#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main()
{
	int grade = 0;
	scanf("%d", &grade);
	int a = grade / 10;
	if (grade <= 100 && grade >= 0)
		switch (a)
		{
		case 10:printf("A"); break;
		case 9:printf("A"); break;
		case 8:printf("B"); break;
		case 7:printf("C"); break;
		case 6:printf("D"); break;
		default:printf("E"); break;
		}
	else
		printf("F");
	return 0;
}